As is known in microlithography, semiconductor structures are produced by the imaging of a structure which has previously been produced on a lithography mask on a wafer that is coated with a photoresist and by exposure and subsequent development of the photoresist.
After the manufacture of the lithography mask, the mask has different properties, which vary from mask to mask. These different properties have a considerable influence on photolithography and on the imaging thereof. In this case, a distinction is drawn between general (global) and local properties which are normally specified.
Global properties are, for example, the error in the line width (CD target value) from the target size (offset error), as well as the fluctuation in the error over the image area (uniformity error). In alternating phase masks, there are structures which have phase-shifting properties when illuminated with coherent light. These phase shifts are produced by etching the relevant structures into the glass. Undesirable brightness errors occur due to discrepancies from the ideal etching depth, and these likewise represent a global mask property.
Local properties are defects on the mask which occur either due to faults in the production of the mask or due to subsequent impurities, e.g., dust.
The aim when producing lithography masks is to produce a lithography mask that satisfies a number of requirements. With respect to the local properties, currently the masks are tested by process inspection and quality measurements.
At present, systematic manufacturing faults in lithography masks such as regular errors in the line separation (offset target error) and changes in the line width (uniformity error) are measured directly on the structures by means of different SEM measurements (Scanning Electron Microscopy) at a large number of points on the mask. However, this is a time-consuming and unreliable assessment process, since the measurement errors and the poor reproducibility of SEM measurements are disadvantageous. For example, the direction of an electron beam significantly influences the measurement result. For this reason, this method is not suitable for lithography masks which are etched in quartz, since there are no well-defined etchings for SEM measurement in masks such as these.
Mask assessment for new mask technologies, such as interference masks, is very difficult. Interference masks contain trenches which have been etched in the mask and which produce diffraction (phase shifting) of the light that is passed through. However, no known method exists for determination of the diffraction and transmission parameters.
A combination of a depth measurement and of an optical measurement using an MSM microscope (Microlithography Simulation Microscope) is used. The measurement values which are obtained with a microscope such as this are compared with line width measurements of structured wafers in order to obtain information about the value of the newly produced lithography masks.
A further aim is to make improvements in defects inspection. Since some defects on the lithography mask are transferred to the chips formed on the wafer, it is very important to ensure an error-free mask structure. Since each process step during mask manufacture unavoidably generates some defects in the lithography mask, it is necessary to be able to inspect and to repair lithography masks.
During mask inspection, the lithography mask is examined for defects, and the defects which are found are classified on the basis of their importance for the lithography and their influence on the functionality of the chip. Mask defects which endanger the functionality and the reliability of the chip function must necessarily be repaired.
The test devices which are currently used employ laser scanning microscopy in order to check lithography masks for defects. In this case, the mask surface is scanned with a laser beam, and the reflected and transmitted light is measured. In defect-free areas, the sum of the reflected light and of the transmitted light is greater than a predefined value. In contrast, in the event of defects, the light is scattered, so that the sum of the intensities is less than the predefined value.
In the case of high-end masks, this approximation is extended in various ways, for example by using an adjacent chip as a reference for the “die-to-die” inspection rather than comparison with a constant reference signal (“die-to-database”).
For standard quality masks, this approximation is simple and efficient in terms of the inspection time and the costs. For high-end masks and in particular for interference masks, this approximation suffers from the fact that masks such as these use a different, shorter wavelength for the first lithography step than that used for the final lithography step. However, since a shorter wavelength means poorer resolution, the sensitivity with respect to defects is also less than during the final lithography process.
Another problem with the laser scanning method is that the interference masks (diffraction gratings) are seen very differently during laser scanning and during optical imaging.
One solution to this problem is to simulate the lithographic process by means of a modified microscope. Lithography microscopes of this type are referred to by the product name MSM (Microlithography Simulation Microscope) and are offered, for example, by the company ZEISS. MSM has the advantage of using the same wavelength as that used for the lithography process, the same illumination of the lithography mask, and the same optical presets for the aperture angle of the projection optics.
In contrast to lithographic exposure, in which the image of the lithography mask is reduced in size, MSM magnifies the image of the lithography mask onto a sensor. However, MSM can image only a small section of the mask at one time.
If the lithography mask is scanned step-by-step, it is possible to record images of it which essentially correspond to the intensity with which a wafer would be exposed.
If a fault has a severe influence on the exposure of the resist, this would be evident from the air image which has been recorded by the MSM. However, in order to make it possible to use the microscope for inspection, an error signal is required which signals the defect. The lithography mask must therefore have various identical chip layouts so that the microscope can compare these images in pairs.
The disadvantage of this method is that each image is influenced by general noise, and that the final error image is subject to greater noise.
Another disadvantage of this method is its complexity, since the entire image of a die must be scanned into the memory of the computer, must be adjusted, and must be compared. This necessitates considerable memory volume as well as considerable computation time.
Upon insertion into a mask stage, an image is generally rotated slightly with respect to the sensor. This mask method can determine and correct the rotation of the mask and the magnification factor of the mask with high accuracy. If the magnification and image rotation are known, the Fourier coefficients can be determined. A reconstructed, noise-free image is obtained by back-transformation by means of a Fourier series.